Method of hot rolling uranium metal



March 10, 1959 A. R. KAUFMANN 2,877,149

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800 I000 IEOO I400 I600 I800 E000 INVENTOR. 4/6.: r Q JW v METHOD OF HOT ROLLING URANIUM METAL Albert R. Kaufmann, Lexington, Mass, assignor to the United States of America as represented by the United States Atomic Energy Commission Application May 21, 1946, Serial No. 671,253

1 Claim. (Cl. 148-115) This invention pertains to a method for hot rolling uranium metal.

In the fabrication of uranium metal a method has been sought by means of which the metal may be quickly and efficiently for-med into sound rods, sheets and the like, having a surface finish free from cracks and checks and which will not warp when subjected to heat of approximately 842 F.

It has been proposed to form uranium rods by extrusion. However, such a process has proven unsuccessful except when the metal is in the gamma phase, that is, when the temperature of the uranium is above approximately 1436 F., and rods so produced have been found to warp when subjected to temperatures ranging from 842 F. to 1085 F.

Forging uranium in the gamma phase is feasible, as is forging in the high alpha region of from approximately 1000 F. to 1220 F. But forging is a slow and expensive process when compared with rolling. Attempts to roll the metal have therefore been made.

Cold rolling uranium, when conducted in the lower portion of the alpha range, has proven unsatisfactory as a commercial process because at low temperatures the metal is hard and diflicult to work. Rolling at higher temperatures corresponding to the beta phase of uranium, that is, from approximately 1220 F. to 1436 F. is also unsuccessful since the metal in this phase is brittle. Metal rolled in the beta phase exhibits excessive cracking and surface checking during the rolling operation, and the process is accompanied by undesirable scale losses. When the temperature is further increased to the gamma phase, the metal becomes mushy and is too soft for rolling. Uranium bars rolled in the gamma phase are characterized by an extremely bad surface finish.

It is an object of this invention to provide a method for more rapidly and economically fabricating sound uranium bars and sheets.

It is a further object of this invention to provide a successful method for hot rolling uranium metal while maintaining the metal in the upper alpha phase.

It is a further object to provide a method of rolling uranium metal with a minimum of roll pressure and power.

These and other objects and features of my invention will be apparent from the following detailed description of my invention.

I have found that the foregoing objects may be achieved and sound uranium rods and sheets may be quickly and easily produced by rolling uranium at high alpha temperatures corresponding to approximately 1000 F. to 1220 F. I have further determined that bars produced by my method are free from the surface and other defects present in bars produced by rolling the metal in the beta and gamma phases, and that bars produced in the manner hereinafter set forth will not warp when subjected to heat of approximately 842 F. to 1085 F.

In the illustrative embodiment of my invention:

Fig. 1 is a graph showing the relationship between temperature of specimen in degrees F. and roll pressure re quired to reduce the specimen by .050 when the specimen has been reduced to one half its original thickness. Fig. 1 further compares the behavior of uranium to mild steel.

Fig. 2 is a graph showing the relationship of roll temperature in degrees F. and roll power required to reduce a specimen by .050 when the specimen has been reduced to one half its original thickness. Fig. 2 also compares uranium with mild steel.

In practicing my invention uranium bars are first heated to a temperature within the upper alpha phase region. This may be accomplished by soaking the bars in a bath of molten lead or by heating them in a protective atmosphere such as argon or natural gas. Still another means of bringing the uranium up to temperature is to immerse the metal in a fused salt bath, such as a eutectic mixture of lithium and potassium chlorides. I prefer to continue heating until the temperature of the metal is approximately 1000 F. to 1220 F.

When the temperature of the metal has been raised to the desired degree rolling is begun. After each pass through the rolls the metal may be restored to the rolling temperature if it is desired to have the temperature the same for each pass. This may involve cooling the billet since the temperature of the metal will rise as a result of working by the rolls. However, if the temperature of the billet falls below rolling temperature, the uranium may be reheated. In any event, it is not essential in actual practice to restore the metal to any specific rolling temperature provided the uranium is maintained in the high alpha phase region during the rolling operation. The roll screws may be turned down by an arbitrary amount such as 0.050" after each pass until the desired thickness is reached. The metal rolled by my method may be quenched in water or annealed.

The elfect of rolling temperature on roll pressure is graphically illustratedin Fig. 1. This graph shows the relationship between the temperature of the specimen in degrees F. and the roll pressure required to reduce the specimen by .050" when the specimen has been reduced to one half thickness. It will be observed that when the temperature of the metal is held within the alpha phase region the roll pressure decreases as the temperature increases. But when the temperature rises to the beta phase range the required roll pressure increases by approximately a factor of three.

Similarly Fig. 2 reveals thatroll power decreases as the temperature of the uranium approaches the upper limit of the alpha phase region, whereas in the beta phase the power consumed increases abruptly by approximately a factor of three.

Both Figs. 1 and 2 illustrate the importance of maintaining the temperature of the uranium in the upper alpha phase region during rolling.

My invention may be more clearly illustrated by reference to the following specific examples:

EXAMPLE 1 A specimen (#444-6) of uranium in bar form containing .078% carbon and /2" by 2" in cross section was heated by immersion in molten lead to a temperature of 800 F. and rolled. After each pass the metal was reheated to rolling temperatures and the rolls were turned down .050". Rolling was continued in this manner until the thickness of the uranium was reduced to approximately .2". When the specimen had been reduced to one half thickness a roll pressure of 12+ and a roll power of 27.8 arbitrary units were observed.

After the final pass through the rolls the metal was. quenched in water.

A cold bend test on a 1" diameter revealed that the metal so rolled had a Rockwell A hardness of 66, an

angle of fracture of 45. A tensile test disclosed an ultimate tensile strength of 142,000 lbs. per sq. inch.

EXAMBLE'2 A specimen (#444-) of uranium in bar form containing .078% carbon and /2" by 2 in cross section was heated by immersion in molten lead to a temperature of 1020 F. and rolled. After each pass the metal was reheated to the rolling temperature and the rolls were turned down .050". Rolling was continued in this manner until the thickness of the uranium was reduced to approximately .2". When the specimen had been reduced to one half thickness, a roll pressure of 5.6 and a roll power of 12.2 were observed.

After rolling, the metal was quenched in water.

A cold bend test on a 1" diameter revealed that the metal had a Rockwell A hardness of 62, and an angle of fracture of 74. A tensile test indicated an ultimate tensile strength of 115,000 lbs. per sq. inch.

s duced to one half thickness a roll pressure of 3 units and a roll power of 7 units was observed.

After rolling, the metal was quenched in water. Subsequent tests disclosed that the metal had a Rockwell A hardness of 56. When subjected to a cold bend test on a 1" diameter the specimen bent 180 without fracturing.

Specimens of uranium metal rolled by my method were subjected to bend tests to determine the relative ductility of metal rolled at ditferent temperatures within the alpha region. Such specimens were cut from uranium strip and were .21" X 1.5" x 4" in size. The tests, performed in an Olsen machine, revealed that the ductility of uranium metal of a given carbon content increases as the rolling temperature approaches the upper limit of the alpha region whereas the reverse is true with respect to hardness. The tests further show that metal which has been annealed after rolling is more brittle than metal which has been water quenched. The results of the bend tests are briefly summarized in the following table:

Table I COLD BEND TESTS-1 DIAMETER [Specimen dimensions-.21" x 1.5" x 4"] Carbon Rolling Anneal- Hardness, Angle of Specimen No. Content, Temp., ing Rockwell Fracture, Remarks Percent F. Tefinp A Degrees 66 63 63 93 59 60 1,500 anneal, lime cool before rolling. 62 74 63 19 58 71 56 180 No fracture. G0 23 Annealed hr. cooled in lime. 59 23 D0. 57 31 Do. 65 21 D0. 58 48 Do. 55 Do. 57 50 Annealed 36 hr. cooled in lime-coarse grained fracture.

EXAMPLE 3 A specimen (#4447) of uranium in bar form con taining 078% carbon and A by 2" in cross section was heated by immersion in molten lead to a temperature of 1180 F. and rolled. After each pass through the rolls the metal was reheated to the rolling temperature and the rolls were turned down .050". Rolling was continued in this manner until the metal was reduced to approximately .2". When the specimen had been re- With respect to tensile strength the efiect of rolling at different temperatures within the alpha phase region is summarized in the following table. The, data show, in general, a marked increase in tensile strength when the specimen is first subjected to cold rolling. The hot rolled tensile specimens were about .211" x /8" at the reduced section, while the cold rolled specimens were approximately .1" X .5. All specimens were cut from 0.2" strips that had been rolled at various stated temperatures in the alpha phase region as previously de-' 1 Specimen contained 0.20% carbon.

While I prefer to reheat the uranium before each pass through the rolls in order to maintain the metal at high alpha temperatures, this step is not essential and may be omitted if desired. However, from the aspect of economy of roll power and pressure it is desirable to assure the maintenance of high alpha temperatures.

Temperature in the beta range should be avoided because of the bad results obtained when uranium metal is rolled in that phase. Inasmuch as rolling will cause an increase in metal temperature, care must be observed when operating at high alpha temperatures lest the temperature of the metal be increased until the beta phase is reached.

By means of my invention a method has been provided whereby uranium metal may be quickly and easily rolled into sound rods, bars and sheets having good surface finish and being free from warping when heated to temperatures of approximately 842 F.

It will be understood that I intend to include variations and modifications of the invention and that the examples herein set forth are illustrative only, and are not to be construed as limitations upon the invention, the scope of which is defined in the appended claim, wherein I claim:

The method of rolling uranium metal that comprises immersing the metal in a bath of fused lead until the temperature of the uranium reaches approximately 1000 F. to 1220 F., passing the heated metal through rolls, restoring the metal to a temperature of approximately 1000 F. to 1220 F. between successive passes of the metal through the rolls, turning down the rolls approximately 0.050" between successive passes of the metal therethrough, and quenching the metal after the final pass through the rolls.

References Cited in the file of this patent UNITED STATES PATENTS 1,082,933 Coolidge Dec. 30, 1913 1,111,698 Liebmann Sept. 22, 1914 2,029,728 Lowry et a1. Feb. 4, 1936 

